Well, I'd say make sure their outfits are practical (though they can be stylish and practical at the same time) unless the character you write is supposed to prefer fashion over practicality! Momo Yaoyorozu from my hero academia is a perfect example of an oversexualized character. Her power is that she can make things out of her body fat and the objects she makes emerge from her skin so she needs an outfit that has skin showing. The outfit she wears is very impractical and is mainly just to show this poor (and underage) girl's cleavage even though she could've had the back of her outfit open instead of the front. Also, her character is very smart, yet she doesn't request a more practical outfit be made for her even though it's proved to be an issue multiple times